The present invention relates to an N-point-converter circuit, and more particularly to an N-point-converter circuit with two power converter valves electrically connected in series that offer an improved performance with respect to voltage breakdown.
High-power drives with an adjustable rotation speed utilize not only DC drives, but increasingly three-phase drives with a line-controlled direct inverter and a machine-controlled converter motor. The limited rotation speed range of the direct inverter drive as well as the limited quality of the three-phase torque (expected torque and dynamics) of converter motors have hitherto prevented more widespread applications of three-phase drives which require less maintenance and which are more robust. Although these limitations or disadvantages could be overcome by using U-inverters, the problems associated with a limited power range remained. Commercially available turn-off semiconductor switches have made it now possible to exceed the megawatt limit. Components with 4.5 kV blocking voltage and 3 kA maximum switchable current are now used in practical applications, making possible inverters with a power of up to 2.5 MW while using only six turn-off semiconductor switches.
To increase the power limits further, a transition to a series and/or parallel connection of turn-off semiconductor switches would be required in conventional U-inverter circuits. This could lead to additional technical problems, significantly increased complexity and higher losses, in particular with circuit networks that symmetrize voltage and current.
One alternative could be a decoupled series connection of two rectifier valves akin to the concept used in three-point inverters. This would double the achievable power range, whereby each turn-off semiconductor switch can be fully utilized by providing two additional diodes. In addition, the output voltage has a significantly improved curve form even at a reduced pulse frequency, which increases the efficiency and reduces harmonic content of the current as well as of the torque.
A three-point converter circuit is known from the publication “Medium Voltage Inverter using High-Voltage IGBTs” by A. Mertens, M. Bruckmann, R. Sommer, published in EPE '99—Lausanne. This three-point converter circuit has two series-connected converter valves, each having two turn-off semiconductor switches, and a voltage intermediate circuit with two capacitors that are electrically connected in series. The connection point of two turn-off semiconductor switches of a respective converter valve is electrically connected through a neutral point diode with the connection point of the two capacitors of the voltage intermediate circuit. This connection point forms of the center of the three-point converter circuit. The turn-off semiconductor switches are implemented as Insulated-Gate-Bipolar-Transistors (IGBT). The three-point converter circuit is capable of producing an output voltage of 2.3 kV using 3.3 kV IGBTs. Four semiconductor switches are used in each converter valve to generate an output voltage of 3.3 kV or 4.16 kV. In other words, instead of using turn-off semiconductor switches with the series connection number ONE, semiconductor switches with a series connection number TWO are used, due to their high blocking voltage.
Moreover, the neutral point diodes are replaced by semiconductor switches with the series connection number TWO. Neutral Point Clamped (NPC) three-point converter circuits are commercially available.
This NPC inverter configuration has the following disadvantages:
                The neutral point is connected by neutral point diodes. A series connection of the components is required to further increase the operating voltage. The series connection of the diodes is not without problems. As long as an IGBT is connected in parallel with the diode, the voltage at the diode is limited by the active measures of the IGBTs. The neutral point diodes require additional circuitry.        The additional circuit elements of the neutral point diodes add other difficulties. When the inner valves are switched on, the charge from the additional circuit elements is discharged into the free-running circuit, which consists of inner valves and neutral point diodes. These free-running currents increase the load on the valves and make it more difficult to employ the circuit.        The implementation of five-point inverters also requires a series connection of the diodes and additional circuit elements.        The commutation sequence has to be observed even in the event of a failure.        
In addition to this NPC inverter configuration, another inverter configurations exists where the neutral point is not clamped. This configuration is referred to as Floating Point (FP). In this FP converter configuration, at least one capacitor is always connected electrically in parallel with two semiconductor switches that are electrically connected in series. The number of the capacitors increases proportionally to the number of the semiconductor switches that are electrically connected in series. I.e., in a five-point converter circuit, ten capacitors are used in the voltage intermediate circuit. The circuit of such five-point converter circuit is known from the publication “The Universal Medium Voltage Adjustable Speed Drive” by Y. Shakweh & E. A. Lewis, published in EPE '99—Lausanne. FIG. 4 in this publication also depicts a five-point converter circuit with an NPC topology. The diagram indicates that 3×4 clamping diodes are used. These diodes can be used to connect the AC side of this five-point converter circuit with the five potentials of the voltage intermediate circuit.
The FP inverter configuration also has disadvantages:                A special control process is required for charging internal capacitors.        A large number of capacitors are required.        
The publication “The Universal Medium Voltage Adjustable Speed Drive” by Y. Shakweh & E. A. Lewis, published in EPE '99—Lausanne, describes another possibility for a medium voltage inverter, whereby several so-called H-bridge rectifiers are electrically connected in series. Such circuit is also referred to as a multi-level cascade, but has the following disadvantages:                A supply with a separate potential is required for each H-bridge. This necessitates complex transformers with several windings.        
The publication “A New Multilevel Inverter Topology with a Hybrid Approach”, by Bum-Seok Suh, Yo-Han Lee, Dong-Seok Hyun and Thomas A. Lipo, published in EPE '99—Lausanne, describes a modified multi-level cascade. This modification enables an even-numbered multipoint topology.
It would therefore be desirable and advantageous to provide a multipoint converter circuit, which obviates prior art shortcomings and can be easily implemented, which has an improved output voltage quality and includes emergency running features.